1. Field of the Invention
The present invention relates to an apparatus and a method for directly and stably driving an oscillation polarization selective light source, such as a distributed feedback (DFB) semiconductor laser that can switch the polarization mode of oscillation light by a modulation current injected thereinto, and an optical communication system using this apparatus or method and the like.
2. Related Background Art
In recent years, increased transmission capacity in the field of optical communications has become desirable, and the development of optical frequency division multiplex (FDM) communication, in which signals at a plurality of optical frequencies are multiplexed in a single optical fiber, has been advanced.
There are two kinds of optical FDM communication methods, which are classified by the type of light signal used in the receiving technique. One method is a coherent optical communication in which a beat signal is produced between signal light and light from a local oscillator to obtain an intermediate-frequency output which output is detected. The other method is one in which only light at a desired wavelength or optical frequency is selected by a tunable filter, and the thus-selected light is detected. The latter method, known as an optical-frequency tunable filtering method, will be described.
The tunable filter may comprise one of a Mach-Zehnder type, a fiber Fabry-Perot type, an acousto-optic (AO) type, a semiconductor filter type and the like, which have been respectively developed.
In the Mach-Zehnder filter type and the fiber Fabry-Perot filter type, the transmission bandwidth can be relatively freely designed and a width of several .ANG. can be obtained, so that the frequency multiplicity of optical FDM communication can be increased. Further, there is a great advantage in that the polarization state of signal light does not adversely affect the quality of the received signal. An example of a Mach-Zehnder type filter is disclosed in K. Oda et al. "Channel Selection Characteristics of Optical FDM Filter", OCS 89-65, 1989. An example of a fiber Fabry-Perot type filter is disclosed in I. P. Kaminow et al. "FDMA-FSK Star Network with a Tunable Optical Filter Demultiplexer", IEEE J. Lightwave Technol., vol. 6, No. 9, p. 1406, September, 1988. Those filter types, however, have the disadvantages that considerable light loss occurs and that downsizing of a receiver device is difficult because the integration of a semiconductor photodetector and the filter is not possible.
In the AO modulator filter type, the receiving control is performed easily since the transmission bandwidth is large, e.g., several tens of .ANG., but the multiplicity of transmitted wavelengths cannot be increased. An example of an AO modulator type filter is disclosed in N. Shimosaka et el. "A photonic wavelength division/time division hybrid multiplexed network using accoustic tunable wavelength filters for a broadcasting studio application", OCS 91-83, 1991. This filter type, however, has the drawbacks that light loss occurs, that the integration with a semiconductor photodetector is not possible and that polarization control of signal light is necessary because the polarization state of signal light adversely affects the quality of the received signal.
In the semiconductor filter type, e.g., a distributed feedback (DFB) filter provided with a diffraction grating formed in a light guide layer for single longitudinal mode operation, the transmission bandwidth can be narrowed (e.g., down to about 0.5 .ANG.), optical amplification (approx. 20 dB) exists, the multiplicity of transmitted wavelengths can be increased and the minimum receiving sensitivity can be improved (i.e., the minimum receiving intensity can be reduced). An example of a semiconductor type filter is disclosed in T. Numai et al. "Semiconductor Tunable Wavelength Filter", OQE 88-65, 1988. Further, this type of filter can be formed with the same material as a semiconductor photodetector, so that integration and downsizing are feasible.
On the other hand, in an optical communication system using the above kinds of filters and a semiconductor laser as a light source, the semiconductor laser is required to have stable oscillation and polarization direction and to maintain a dynamic single mode. Therefore, a DFB laser, a distributed Bragg reflector (DBR) laser, or the like is used as a semiconductor laser since each radiates in only the transverse electric (TE) mode. At present, the most popular modulation system for transmission signals in transmission systems is digital amplitude modulation, or amplitude shift keying (ASK) in which a drive current injected into a laser is directly modulated, or digital frequency modulation or frequency shift keying (FSK) in which a signal current having a minute amplitude is superposed on a bias current.
In the FSK system, techniques have been developed, for example, for using the wavelength discrimination function of an optical filter to demodulate signals. In this connection, reference should be made to M. J. Chawki et al. "1.5 Gbit/s FSK Transmission System Using Two Electrode DFB Laser As A Tunable FSK Discriminator/Photodetector", Electron. Lett. vol. 26 No. 15, p. 1146, 1990.
Furthermore, another system has been proposed in which the polarization mode of oscillated light from a DFB laser is switched between light in TE and TM (transverse magnetic) modes and only one of TE and TM modes is selected (see, for example, Japanese Patent Laid-Open No.2(Heisei)-159781). When an ordinary DFB laser is used, however, it is difficult to reduce a modulated drive current below 10 mA because there is a great difference in gain of its active layer between TE and TM modes. Thus, dynamic wavelength fluctuation is not reduced much, even compared with the ASK system.
Further, in the direct optical intensity or amplitude modulation system, the spectral line width is widened to about 0.3 nm due to the dynamic wavelength fluctuation. In addition, the wavelength tunable width is typically approximately 3 nm, so that the number of channels cannot be made large and hence the direct ASK modulation system is unsuitable for frequency or wavelength division multiplexing transmission (generally, the optical frequency or wavelength interval between adjacent channels in the frequency division multiplexing transmission is much narrower than that in the wavelength division multiplexing transmission). On the other hand, when an external intensity modulator is used to modulate constant-intensity light from a light source or semiconductor laser, the number of devices will increase and hence this system is disadvantageous in cost even though the wavelength fluctuation can be reduced.
Further, in the direct frequency modulation system, the channel width is narrow and hence the number of channels can be increased. However, the tracking control of a tunable filter needs to be accurately performed. Further, there is a tendency for crosstalk between wavelengths indicating codes "1" and "0" to occur depending on a change in surroundings, and an error rate of received signals increases.
Further, in the polarization modulation system in which the polarization mode is switched, though the modulation can be performed by a minute signal, a polarizer disposed in front of the emission end of a laser and a filter disposed at the side of a receiver are needed to perform signal transmission with deep modulation. Thus, the number of devices and cost cannot be reduced. Furthermore, the extinction ratio is sensitive to a change in a bias current injected into the laser.
The direct polarization modulation system will be described in more detail. FIG. 1 illustrates the system. As illustrated in FIG. 1, the system includes a two-electrode DFB-LD 11-1, an adder 15-1, driving circuits 11-10-1 and 11-10-2, and a polarizer 11-11. In the two-electrode DFB-LD 11-1, the oscillation polarization mode can be switched by changing currents injected through the two electrodes. Such characteristics can be obtained by adjusting device parameters, such as the Bragg wavelength of a diffraction grating and the gain spectrum of an active layer. The driving circuits 11-10-1 and 11-10-2 supply currents corresponding to input signals thereinto, respectively. Those currents are injected into the two-electrode DFB-LD 11-1 through the two electrodes. The adder 15-1 adds two input signals, i.e., a bias signal and a modulation signal, to each other. The output of the adder 15-1 is connected to the driving circuit 11-10-1, and a bias signal is directly input into the driving circuit 11-10-2. The modulation signal and the two bias signals are supplied to the light source apparatus from a transmitter in which the light source apparatus is contained. The polarizer 11-11 only transmits a TE polarization component of output light from the two-electrode DFB-LD 11-1, and the thus-created modulated output (i.e., ASK signal) is transmitted through a transmission line.
FIG. 2 illustrates oscillation characteristics of a two-electrode DFB-LD which can switch its oscillation polarization mode between TE and TM modes. The abscissa indicates a current I.sub.1 injected through a front side electrode, the ordinate indicates a current I.sub.2 injected through a rear side electrode, and a region of TE mode oscillation (right-hand lower portion of the thick dotted line) and a region of TM mode oscillation (left-hand upper portion of the thick dotted line) are shown. Curves in each region indicate contours of output intensities of the respective polarization modes, and the intensity of the output increases along an arrow.
FIG. 3 illustrates a manner of oscillation switching between TE and TM modes. FIG. 3 illustrates changes in light intensities of respective polarization modes in the case when the current injected through the front electrode is fixed to I.sub.1b and the current I.sub.2 injected through the rear electrode is changed. A portion near a switching region (described below) is illustrated in enlarged form. The oscillation occurs in only the TE mode when I.sub.2 &lt;I.sub.2smin, the oscillation occurs in both TE and TM modes when I.sub.2smin &lt;I.sub.2 &lt;I.sub.2smax, and the oscillation occurs in only the TM mode when I.sub.2 &gt;I.sub.2smax. When I.sub.2smin &lt;I.sub.2 &lt;I.sub.2smax, the oscillation condition is unstable both in the TE and TM modes, and the time-averaged light intensity of the TE mode decreases and that of the TM mode increases as the current I.sub.2 increases. At a point of I.sub.2sc, the light intensity of the TE mode is equal to that of the TM mode. In the following explanation, the region between I.sub.2smin and I.sub.2smax is referred to as the switching region.
The direct polarization modulation can be achieved by setting a bias current point below the switching region and superposing a modulated current thereon. For example, I.sub.1 can be fixed and I.sub.2 can be modulated. First, I.sub.1 is fixed at I.sub.1b. The switching between TE and TM modes occurs when I.sub.2 is changed in a range between a value below I.sub.2smin and a value above I.sub.2smax. The bias component I.sub.2b and the modulation component I.sub.2m of I.sub.2 are set such that the condition I.sub.2b &lt;I.sub.2smin and I.sub.2b +I.sub.2m &gt;I.sub.2smax is satisfied. Thus, the oscillation takes place in the TE mode when I.sub.2 =I.sub.2b, while the oscillation takes place in the TM mode when I.sub.2 =I.sub.2b +I.sub.2m. The polarizer 11-11 takes out only the TE mode component of the light signal, and an intensity-modulated light signal is obtained. In this example, the light output is ON when I.sub.2 =I.sub.2b, and the light output is OFF when I.sub.2 =I.sub.2b +I.sub.2m. Thus, the modulation signal is inverted by the light source apparatus illustrated in FIG. 1.
In the modulation system, the DFB-LD is directly modulated, so the structure thereof is as simple as the direct FSK modulation system. Further, the amplitude of the modulation current is small, say several mA, and the laser is always oscillated in both states corresponding to mark and space of the FSK modulation signal. Therefore, optical frequency chirping of the oscillated light signal obtained by the modulation is as small as the external modulation system.
The direct polarization modulation can also be obtained by setting a bias point of injection current in the switching region and modulating the current with an appropriate modulation amplitude. This case will be described using FIGS. 2, 5 and 6A-6C. I.sub.1 is fixed at I.sub.1b. I.sub.2 is a square waveform whose bias component is I.sub.2b and whose modulation component has an amplitude I.sub.mod. The upper end of this square waveform is I.sub.mod /2, and the lower end of this square waveform is -I.sub.mod /2. They respectively correspond to mark and space of FSK. I.sub.2b and I.sub.mod are set such that only the TE mode is oscillated when I.sub.2E =I.sub.2b -I.sub.mod /2 and only the TM mode is oscillated when I.sub.2M =I.sub.2b +I.sub.mod /2 (see FIGS. 6A and 6B). The intensity-modulated light signal can be produced by selecting either the TE or TM polarization component using the polarizer 11-11 (see FIG. 6C). In the following description, when "bias component" or "bias point" is refered to, the above-discussed two meanings (the case where the modulation component is considered to be DC-like and the case where the modulation component is considered to be AC-like) are used discriminately.
The direct polarization modulation system has the following disadvantage. The distribution of the TE and TM oscillation regions of a multi-electrode DFB-LD varies for each device. Therefore, the bias point of the direct polarization modulation needs to be set on the basis of precise measurements for each device. Further, the distribution of the TE and TM oscillation regions changes due to temperature and the like even in the same device. Hence, it is difficult to maintain the state of polarization modulation (e.g., an intensity ratio between TE and TM modes that corrensponds to the modulation signal).
Impairment of the state of polarization modulation due to some change in a device will be described using FIGS. 3 and 4A-4C. FIGS. 4A-4C illustrate light outputs when the bias component of injection current is appropriate and when it deviates from an optimum switching point.
In FIG. 3, the bias component I.sub.2b and the modulation component I.sub.2m of I.sub.2 are initially set such that I.sub.2b &lt;I.sub.2smin -.delta. and I.sub.2b +I.sub.2m &gt;I.sub.2smax +.delta. are satisfied. The magnitude of .delta. is minute. In the two-electrode DFB-LD 11-1 under such a condition, only TE mode light is emitted when I.sub.2 =I.sub.2b and only TM mode light is emitted when I.sub.2 =I.sub.2b +I.sub.2m. The intensity-modulated light signal produced by the polarizer 11-11 changes as shown in FIG. 4A. Since the light source inverts the modulation signal, the intensity of the light signal is high when I.sub.2 =I.sub.2b and low I.sub.2 =I.sub.2b +I.sub.2m.
When current values I.sub.2smin, I.sub.2smax and I.sub.2sc of the switching region vary and I.sub.2b exceeds I.sub.2smin, the light intensity of the TE mode at the time of I.sub.2 =I.sub.2b decreases, whereas the light intensity of the TM mode increases. Further, when I.sub.2b exceeds I.sub.2smax, only the TM mode begins to be oscillated. This is illustrated in FIG. 4B. In comparison, when I.sub.2b becomes lower than I.sub.2smax -I.sub.2m, the light intensity of the TM mode at the time of I.sub.2 =I.sub.2b +I.sub.2m, decreases and the light intensity of TE mode increases. Further, when I.sub.2b becomes less than I.sub.2smin -I.sub.2m, only the TE mode begins to be oscillated. This is illustrated in FIG. 4C.
As is apparent from the foregoing, when current values of the switching region deviate, the modulation efficiency of polarization modulation is impaired (see FIG. 4C), or in some cases no correct modulation will be achieved (see FIG. 4B).